Rapidly growing markets for wireless voice, data, and multimedia services, as well as expanding applications, will place severe demands upon radio frequency spectrum in the next few years. In order to meet the projected demand, physical layer communications equipment will require higher bandwidth efficiency than is achievable today. In addition, it is expected that the cost and power consumption of mobile equipment will remain at least where they are today, these features being particularly important in hand held radios, where size and power supplies are limited.
Quadrature amplitude modulation(“QAM”) may be used to address the need for high spectral efficiency but the envelope of the transmitted signal is not constant and its use with non linear (low cost) power amplifiers opens up problems of spectral regrowth, adjacent channel interference, and intersymbol interference. When used with non-linear power amplifiers QAM is sometimes used with the predistortion-based amplifier linearization (at the transmitter) and equalization (at the receiver) however both increase cost and power consumption. A known approach is to avoid the nonlinearity by using a continuous phase modulation waveform (“CPM”) which allows the transmission of many bits per symbol while maintaining a constant envelope suitable for linear and non-linear amplification. Constant envelope waveforms such a CPM are ideally suited for high efficiency, low power amplifiers because of the negligible peak to average ratio that allows non-linear amplifiers to be used for this application.
In many lower power communication systems compromises are made reducing the size of the power amplifier and minimizing waveform complexity. It is desired that many existing waveforms be extended across all of the various radio platforms from high power high frequency transmitters to low power handheld radios. Traditionally, the size of the handheld radio has limited its ultimate power output and receiver complexity.
CPM waveforms are typically demodulated in super heterodyne receivers. Super heterodyne receivers require the signal to be within a certain dynamic range, to this end they employ automatic gain controllers (“AGC”) which consumes physical space and power within the receiver. Such receivers also must utilize fast analog to digital converters (“ADC”) which can limit the symbol rate of the received signal and also consume limited physical space and power.
Embodiments of the present invention, as an alternative to prior art systems, use a pulse count demodulator (“PCD”) for CPM waveform demodulation and information extraction. While pulse count demodulation has been used for FM voice and non-coherent binary frequency shift keyed systems they have not previously been applied to CPM waveforms. The embodiments of the present invention apply in a coherent system where the incoming frequency samples are reinterpreted and integrated to provide a large performance gain associated with CPM waveforms.
Thus the pulse count demodulator according to embodiments of the present invention is ideal for the handheld environment, many single chip solutions are available and pulse count demodulators do not require automatic gain control circuits to provide dynamic range or fast ADCs anywhere in the receive path. This results in a decrease in size, cost and power requirements of the RF portion of the receiver while maintaining the advantages attributed to CPM waveforms. Hand held radio receivers for square wave CPM waveforms in the UHF frequency band as specified in MIL-STD-188-181B, incorporated by reference herein, and used predominately in military systems are especially suitable for employing a pulse count demodulator as described in the embodiments herein. An article entitled “Low Power Method for demodulation of continuous Phase Modulated waveforms.” MILCOM2002, Oct. 7, 2002, by the inventor presents simulated results of a PCD with CPM wave forms. The contents of which are hereby incorporated by reference.
For these and other reasons it is an object of the present invention to provide a novel improvement to a mobile radio receiver for receiving a continuous phase modulation CPM waveform containing information symbols. For a mobile radio receiver comprising a super heterodyne demodulator, an automatic gain controller and a high rate analog to digital converter, one embodiment of the present invention replaces the super heterodyne demodulator, AGC and ADC with a pulse count demodulator for demodulating the CPM waveform to extract the information symbols.
It is also an object of the present invention to provide a novel hand held radio system with a limited power supply for receiving information from a continuous phase modulation waveform signal. One embodiment of the present invention comprises a hand held radio system including a delay circuit delaying the CPM signal by a time increment t0 and a subtractor creating a difference from the CPM signal and the delayed CPM signal. The radio system also includes a half-wave rectifier for rectifying the difference and a low pass filter integrating the rectified difference to obtain an average signal. Where the amplitude of the average signal represents a phase change from which received information can be extracted.
It is still an object of the present invention to provide a novel low power method of demodulating a constant envelope waveform signal. Embodiments of the method include the steps of providing a constant envelope waveform signal containing information symbols, clipping the constant envelope waveform to create a square wave signal, and delaying the square wave signal in a delay circuit to obtain a time-delayed square wave signal. The time-delayed square wave signal is subtracted from the square wave signal to create a difference signal. The embodiments also include rectifying the difference signal to create a half wave rectified signal, integrating the rectified signal to create an average signal, and extracting the information symbols from the constant envelope waveform based on the amplitude of the average signal.
It is yet another object of the present invention to provide an improved method of demodulating a constant envelope waveform encoded with information symbols. An embodiment of the present invention includes the step of demodulating the constant envelope waveform with a pulse count demodulator to extract the information symbols.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments